Investments (Cost And Estimated Fair Value Of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 7,736	$ 7,741
Gross unrealized gain	23	24
Gross unrealized loss	(9)	(9)
Fair value, investments	7,750	7,756
Cash equivalents, Carrying amount	601	963	$ 670	$ 840
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	346	568
Gross unrealized gain	0	0
Gross unrealized loss	0	0
Fair value, investments	346	568
Treasury Bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	7,682	7,687
Gross unrealized gain	23	24
Gross unrealized loss	0	0
Fair value, investments	7,705	7,711
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	54	54
Gross unrealized gain	0	0
Gross unrealized loss	(9)	(9)
Fair value, investments	45	45
Debt and Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	8,082	8,309
Gross unrealized gain	23	24
Gross unrealized loss	(9)	(9)
Fair value, investments	8,096	8,324
Less Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gain	0	0
Gross unrealized loss	0	0
Cash equivalents, Carrying amount	346	568
Cash equivalents, Fair value	$ 346	$ 568